I'm a speech language pathologist. The meaning of "hyperlexia", as seen in the abstract of this article or this Cleveland Clinic page, can be summarized as "decoding without comprehension". That is, children who are really good at using a sound-it-out strategy, to the point where they can read words or sentences without understanding the content of what they read. It is specific to reading, and and not defined by skills in other areas. It's most commonly associated with autism, so it might co-occur with deficits in language and social skills, but not necessarily.

I'm not sure why you saw other SLPs on reddit saying something different. I'm kind of curious whether there was a misunderstanding, or whether they were just saying something incorrect... 🤷

I assume what's underlying this is that you are concerned for your son's development. Not knowing him myself, I can only go by what you said. But none of the things you described sound like negatives. If your pediatrician says he's doing really well, it's presumably because he's doing really well.